Population size predicts technological complexity in Oceania

Here is a far-reaching and crucially relevant question for those of us seeking to understand the evolution of culture: Is there any relationship between population size and tool kit diversity or complexity? This question is important because, if met with an affirmative answer, then the emergence of modern human culture may be explained by changes in population size, rather than a species-wide cognitive explosion. Some attempts at an answer have led to models which make certain predictions about what we expect to see when populations vary. For instance, Shennan (2001) argues that in smaller populations, the number of people adopting a particular cultural variant is more likely to be affected by sampling variation. So in larger populations, learners potentially have access to a greater number of experts, which means adaptive variants are less likely to be lost by chance (Henrich, 2004).

Models aside, and existing empirical evidence is limited with the results being mixed. I previously mentioned the gradual loss of complexity in Tasmanian tool kits after the population was isolated from mainland Australia. Elsewhere, Golden (2006) highlighted the case of isolated Polar Inuit, who lost kayaks, the bow and arrow and other technologies when their knowledgeable experts were wiped out during a plague.Yet two systematic studies (Collard et al., 2005; Read, 2008) of the Inuit case found no evidence for population size being a predictor of technological complexity.

In the current paper, Kline & Boyd (2010) investigate the effects of population size on the complexity of marine foraging tool kits among island populations in Oceania. Importantly, they consider contact between populations:

However, the sample used in both analyses did not include any measure of contact between populations and was drawn mostly from northern coastal regions of the western North America where intergroup contact was probably common (Balikci 1970; Jordan 2009), but difficult to estimate. If, as the cultural adaptation models predict, frequent contact between groups mitigates the effects of small population size, then the results from these analyses do not provide a test of the models.

Using the electronic Human Relations Area Files (eHRAF), Kline & Boyd take a sample of information on indigenous marine foraging tool kits from 10 island societies. This also includes data on the rates of contact between populations and controls for aspects such as resource failure. The general results support the hypothesis in three ways. First, large populations retain a larger repertoire of tools than small island populations (see graph below) — with population size being a much better predictor than other explanatory variables. Second, there is some support for the prediction that contact will be less important in larger populations. For instance, four of the five high-contact societies have more tool types than expected given that they fall within the intermediate range of population sizes. Conversely, low contact groups displayed a trend of having fewer tools than expected, with the overall predictive power of population size and contact being second to population size and fish genera.

Their third and final point concerns how complex tools will be particularly prone to loss, due to how much harder it is to learn and make them. To quantify tool complexity, the authors used techno-units: defined as “an integrated, physically distinct and unique structural configuration that contributes to the form of a finished artefact”. As an example, on one end of the scale (one techno-unit) there is a stick used for prying shellfish, whilst an untended crab trap utilising a baited lever is on the end end with 16 techno-units. Applied to the current dataset, and the mean number of techno-units is significantly higher in large populations than in smaller populations.

If Kline & Boyd’s view of human adaptation is correct, then the constraints imposed on cultural adaptation by population size and rate of contact are more relevant determinants than ecological factors (in this case at least). As they note:

To test this hypothesis, we chose to study island populations because they are ecologically similar and because population size and contact rates are easier to estimate than in continental populations. Then by limiting the analysis to marine foraging tools, we hoped to minimize the effects of ecological variation on tool kit complexity. Thus, our observation that larger populations have more kinds of marine foraging tools and more complex tools than smaller, isolated populations supports the hypothesis that gradual cultural evolution plays an important role in human adaptation.

There are alternative explanations for the relationship between population size and tool kit complexity. It could be the case that more complex marine foraging technology merely increases the local carrying capacity, subsequently allowing for larger population sizes. However, this explanation does not explain the relationship between population contact and tool kit complexity. Yet there are some aspects of the study I would be cautious about. Take the data they used to measure the rate of contact between populations: it only distinguishes between high or low levels of contact. These limitations certainly set the stage for a more extensive study; using finer-grained measures of contact. But as the authors themselves conclude:

These findings are a first step in understanding the nature of cumulative cultural gains and losses. Although our sample size is small and our analysis is restricted to a limited range of tool types, our results suggest that cultural drift of the treadmill mechanism may have influenced the evolution and adaptive radiation of Homo sapiens as a cultural species.

An interesting study – I wonder how many techo-units there are in my laptop.

However, I’m wondering whether population size and levels of contact with other groups can be separated. How are groups defined if not by frequent contact? If you’re in frequent contact with a person, does that make them part of your group? Also, the more people in your group, the more contact you’re able to have with other groups.

That is, group size and levels of contact may be correlated, so the results here aren’t that compelling.

Haha, probably more units than my PC, which is currently in permanent hibernation. I wouldn’t be surprised if Techno-Units was the name of a electro-pop band…

But anyway, I agree that population size and levels of contact are hard to separate. This is why Kline and Boyd studied the island populations of Oceania. As they themselves state: “Because island populations are geographically bounded and separated by significant distances, it is possible to estimate population sizes and contact rates with a reasonable degree of accuracy.”

I can see where you’re going with these questions. But, if we agree that a group is a distinct population, in the sense that they show a distinct culture and, perhaps, a distinct genetic profile, then you would expect to see instances where high-contact societies have more tool types than expected based on their population size.

…And this is exactly what the authors found: “four of the five high-contact societies have more tool types than expected based on their population size. These five societies fall in the intermediate range of population size.”

So, whilst I agree group size and levels of contact may generally be correlated, I can also imagine certain instances where smaller groups, with large levels of contact, might display levels of technological sophistication that population size alone cannot explain.

Godfrey-Smith talks about Darwinian populations. What he seems to be suggesting is that the level at which evolution operates on changes as the strengths of various properties of the population change. These properties include the fidelity of heritability, the level of extrinsic influence on reproduction and the smoothness of the adaptive landscape.

Cultural traits such as technology are very different to genetic/pheonotypic traits in some ways – the fidelity of heritability is lower and a small change can give an individual a large adaptive advantage (think about lethal weapons). Although I haven’t read the whole book yet, I think Godfrey-Smith is suggesting that this means that the Darwinian Population (the group of things that evolve) changes, from individuals with genes to, perhaps, island-sized groups with techo-units.

I’m sure that Kline & Boyd and others would agree with this, but it’s an interesting idea that a population may be defined by the evolutionary pressures working inside it.